I. Technical Field
The present invention relates to an electret electrode, a static induction vibration power generator, an actuator and vibration power generating device using the electret electrode, and an electronic equipment using the vibration power generating device, and a communication device using the vibration power generating device.
II. Description of the Related Art
There has already been proposed a static induction vibration power generating device in which an electric charge is applied to one electrode of a variable capacity and the electric charge is induced to an opposed electrode through electrostatic induction, and then the electric charge to be induced is varied by changing of capacity and the change in the electric charge is taken out as electric energy (see, for example, JP2005-529574A (pages 10-11. FIG. 4)).
FIG. 7 shows the static induction vibration power generator described in aforementioned JP2005-529574A (pages 10-11, FIG. 4). FIG. 7 is a schematic sectional view of a vibration power generator 10 using an electret.
This static induction power generator is constituted from a first substrate 11 equipped with a plurality of conductive surface regions 13, and a second substrate 16 equipped with a plurality of electret material regions 15. The first substrate 11 and the second substrate 16 are mutually disposed at a predetermined distance. The second substrate 16 including the electret material regions 15 is fixed. The first substrate 11 including the conductive surface regions 13 are connected to a fixed structure 17 through spring 19. The spring 19 is connected to both side surfaces of the first substrate 11 and is also connected to the fixed structure 17. This spring 19 makes the first substrate 11 to return to the fixed position, or the first substrate can return to the fixed position by carrying out a lateral movement (for example, movement in an X-axis direction). This movement causes a variation in an overlapped area between the electret material regions 15 and opposed conductive surface regions 13, and thus a change in electric charge arises in the conductive surface regions 13. According to the static induction power generator, power generation is carried out by taking a change in electric charge as electric energy.
At this time, maximum output power Pmax is represented by the following equation:
                              P          max                =                                            σ              2                        ⁢            nAf                                4            ⁢                                                            ɛ                  Electret                                ⁢                                  ɛ                  0                                            d                        ⁢                          (                              1                +                                                      g                    ⁢                                                                                  ⁢                                          ɛ                      Electret                                                                            d                    ⁢                                                                                  ⁢                                          ɛ                      air                                                                                  )                                                          [                  Equation          ⁢                                          ⁢          1                ]            where σ denotes a surface electric charge (density), ∈Electret denotes a dielectric constant of an electret material, ∈air denotes a dielectric constant of air, ∈0 denotes a dielectric constant of vacuum, A denotes an overlapped area between an electret material region and a conductive surface region, g denotes a gap between electrodes, f denotes a vibration frequency, d denotes a film thickness of an electret material, and n denotes the number of overlapped areas.
As is apparent from the equation, it is necessary to increase the surface electric charge (density) of the electret material, that is, a surface potential of the electret material so as to increase a power generation amount.
On the other hand, a silicon oxide film has been known as the electret material (see, for example, TRANSDUCERS & EUROSENSORS '07 The 14th International Conference on Solid-State Sensors, Actuators and Microsystems, Lyon, France, Jun. 10-14, 2007). Also, IEEE Transactions on Dielectrics and Electrical Insulation Vol. 13, No. October 2006 discloses an amount of charge to a silicon oxide film used as the electret material.
FIG. 8 is a schematic sectional view of a conventional static induction vibration power generator described in aforementioned TRANSDUCERS & EUROSENSORS '07 The 14th international Conference on Solid-State Sensors, Actuators and Microsystems, Lyon, France, Jun. 10-14, 2007, that is a generator (static induction vibration power generator) 20 using a silicon oxide film as the electret material. In FIG. 8, a fixed electrode 22 is formed on a glass 21. A suspended mass 24 is disposed on the glass 21 through adhesive bonding. A silicon substrate 26 with an electret 25 formed thereon is disposed on silicon 27 through adhesive bonding.
According to this static induction power generator, power generation is carried out by utilizing the fact that the suspended mass 24 including a movable electrode 23 is vibrated thereby causing a change in capacity of Cvar. In this static induction power generator, a silicon oxide film is used as the electret material. The electret material region is constituted by forming a silicon nitride film, a silicon oxide film and a silicon nitride film on an electrode, and is subjected to a heat treatment so as to stabilize the electric charge.
FIG. 9 is a graph showing a relationship between the time of electron charge (the time of electron charging) to a silicon oxide film and the surface potential described in aforementioned Non-Patent Document 2. The symbol “◯” denotes a relationship between the time of electron charge and the surface potential when the thickness of a silicon oxide film is 0.5 micrometer, while the symbol “Δ” denotes a relationship between the time of electron charge and the surface potential when the thickness of a silicon oxide film is 0.6 micrometer. In both cases, the surface potential of the silicon oxide film increases as the time of electron charge becomes longer. When the surface potential arrives at a maximum value, the surface potential does not increase even when the time of electron charge is prolonged. In the silicon oxide film having a thickness of 0.5 micrometer, the maximum value of the surface potential is 240 V. In the silicon oxide film having a thickness of 0.6 micrometer, the maximum value of the surface potential is 290 V. Aforementioned Non-Patent Document 2 describes that the maximum value of the surface potential depends on a dielectric strength voltage (or breakdown voltage) of the silicon oxide film.
Accordingly, in the static induction vibration power generator using the silicon oxide film as the electret material, it is necessary to increase the dielectric strength voltage of the silicon oxide film, that is, the film thickness so as to improve the maximum output power by increasing the surface electric charge (density) of the electret material.